Neutral Strangeness Production with the ZEUS Detector at HERA Chuanlei Liu Department of Physics McGill University, Montreal, QC Canada January 2007 A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Doctor of Philosophy Chuanlei Liu, 2007 Abstract ¯ 0 The inclusive production of the neutral strange particles, Λ, Λ and KS, has been studied with the ZEUS detector at HERA. The measurement provides a way to understand the fragmentation process in ep collisions and to check the universality of this process. The strangeness cross sections have been measured and compared with Monte Carlo (MC) predictions. Over the kinematic regions of interest, no Λ ¯ 0 to Λ asymmetry was observed. The relative yield of Λ and KS was determined and the result was compared with MC calculations and results from other experiments. A good agreement was found except for the enhancement in the photoproduction process. Clear rapidity correlation was observed for particle pairs where either quark 0 0 flavor or baryon number compensation occurs. The KSKS Bose-Einstein correlation measurement gives a result consistent with those from LEP measurements. The Λ polarizations were measured to be consistent with zero for HERA I data. i ii Abstract Abstract in French ¯ 0 La mesure de production inclusive des particules ´etranges neutres Λ, Λ et KS avec le d´etecteur ZEUS a` HERA permet de comprendre le processus de fragmentation dans les collisions ep et de v´erifier son universalit´e. Les sections efficaces de produc- tion d'´etranget´e ont ´et´e mesur´ees et compar´ees avec les pr´edictions de simulations Monte-Carlo (MC). Dans les r´egions cin´ematiques d'int´er^et, aucune asym´etrie de production entre baryons et anti-baryons n'a ´et´e observ´ee. Les taux relatifs de pro- 0 duction entre les Λ et les KS ont aussi ´et´e mesur´es et compar´es avec les calculs MC et les r´esultats d'autres exp´eriences. L'accord est en g´en´eral bon sauf pour un sur- plus observ´e dans le processus de photoproduction. De plus, des effects prononc´es de corr´elation en rapidit´e ont ´et´e mis en ´evidence pour des paires de particules pour les- quelles la compensation de saveur ou de nombre baryonique est possible. Les ´etudes 0 0 de corr´elation Bose-Einstein du syst`eme KSKS donnent un r´esultat consistant avec celui des exp´eriences du LEP. Enfin, les polarisations des Λ se sont av´er´ees compa- tibles avec z´ero pour les donn´ees de HERA I. Contents 1 Introduction 1 2 Physics Overview 5 2.1 Kinematics of ep Scattering . 5 2.2 Inclusive NC DIS Cross Section . 9 2.3 Quark Parton Model (QPM) . 10 2.4 Improved QPM . 11 2.5 Strangeness Physics . 14 2.5.1 Strangeness Production Mechanism . 14 2.5.2 The Strangeness Suppression Factor . 15 2.6 Physics Motivations of the Thesis . 16 2.6.1 Test of the MC Models . 18 2.6.2 Baryon Number Flow . 18 2.6.3 Baryon Production Enhancement . 19 2.6.4 Dynamics of Hadron Formation . 20 2.6.5 Strange Hadron Polarizations . 22 3 Experimental Setup 25 3.1 The HERA ep Collider . 25 3.2 The ZEUS Detector . 28 3.2.1 The Central Tracking Detector (CTD) . 30 3.2.2 The Uranium Calorimeter (CAL) . 32 3.2.3 Small Angle Rear Tracking Detector (SRTD) . 35 3.2.4 Luminosity Measurement (LUMI) . 35 3.3 Trigger and Data Acquisition System . 36 3.3.1 First Level Trigger (FLT) . 37 iii iv CONTENTS 3.3.2 Second Level Trigger (SLT) . 37 3.3.3 Event Builder (EVB) . 38 3.3.4 Third Level Trigger (TLT) . 38 3.4 Expectations and Challenges for HERA II . 38 4 Event Reconstruction and Selection 39 4.1 Event Reconstruction . 39 4.1.1 Electron Finder . 40 4.1.2 Jet Reconstruction . 40 4.1.3 DIS Event Reconstruction . 42 4.1.4 Photoproduction Event Reconstruction . 44 4.2 Event Selection . 46 4.2.1 Online Preselection . 46 4.2.2 Deep Inelastic Scattering . 48 4.2.3 Breit Frame in DIS . 50 4.2.4 Photoproduction Process . 51 4.2.5 Fireball Events in Photoproduction . 52 5 Strange Particle Reconstruction 57 5.1 Strange Hadrons of Interest . 57 5.2 Particle Reconstruction and Selection . 58 5.2.1 Secondary Scattering Contamination . 59 5.2.2 Λ Reconstruction . 62 0 5.2.3 KS Reconstruction and Selection . 67 5.3 Signal Extraction . 68 5.4 Efficiency, Purity and Acceptance . 69 6 Monte Carlo Simulation 75 6.1 Physics Simulation . 75 6.1.1 Hard Scattering Process . 76 6.1.2 Perturbative QCD Radiation . 77 6.1.3 Fragmentation, Hadronization and Decay . 79 6.1.4 Beam Remnant and Multiple Interaction . 81 6.2 Detector Simulation . 82 Contents v 6.3 MC Simulation for the Thesis . 82 6.3.1 MC Files . 84 6.3.2 Control Plots . 85 7 Results I: Cross Sections and Ratios 97 7.1 Total Cross Section . 98 7.2 Differential Cross Section . 102 7.2.1 Photoproduction Events . 103 7.2.2 Low Q2 DIS Events . 105 7.2.3 High Q2 DIS Events . 108 7.3 Λ to Λ¯ Production Asymmetry . 115 7.3.1 Photoproduction Events . 117 7.3.2 Low Q2 DIS Events . 118 7.3.3 High Q2 DIS Events . 118 0 7.4 Λ to KS Production Ratio . 122 7.4.1 Photoproduction Events . 124 7.4.2 Low Q2 DIS Events . 127 7.4.3 High Q2 DIS Events . 128 8 Results II: Correlations and Polarizations 135 8.1 Bose-Einstein Correlation (BEC) . 136 8.2 Rapidity Correlation (RC) . 140 8.3 Polarization . 147 9 Conclusions and Outlook 153 9.1 Cross Section Measurements . 153 9.2 Baryon to Antibaryon Asymmetry . 154 9.3 Baryon to Meson Ratio . 155 9.4 Bose-Einstein Correlation (BEC) . 157 9.5 Rapidity Correlation (RC) . 158 9.6 Λ Polarizations . 158 9.7 Outlook . 159 Appendices 160 vi CONTENTS A Determination of Systematic Uncertainties 161 A.1 Photoproduction Sample . 162 A.2 DIS Samples . 168 List of Figures 2.1 An illustration of the HERA kinematics for a NC process . 6 2.2 Corresponding phase spaces in e+e−, ep and hadron-hadron collisions 8 2.3 The NC and CC DIS cross section measurements at HERA . 10 2.4 Summary of the F2 measurements from the HERA and fixed target experiments . 12 2.5 Strange production mechanisms in ep collisions . 15 0 2.6 Early ZEUS measurements on KS and Λ production . 17 2.7 Anti-baryon to baryon asymmetries measured at RHIC . 19 2.8 Relative baryon to meson production ratios measured at RHIC exper- iments . 20 2.9 The BEC results as a function of Q2 measured in ZEUS . 21 2.10 The dependence of radius on the hadron mass . 22 2.11 Rapidity correlations on different baryon pairs . 23 3.1 A bird-eye view of the DESY site . 26 3.2 The HERA accelerator, PETRA pre-accelerator and the experiments. 26 3.3 The luminosity delivered by HERA . 27 3.4 Cross section of the ZEUS detector parallel to beam pipe. 29 3.5 Cross section of the ZEUS detector perpendicular to beam pipe. 29 3.6 Cross section of one ZEUS CTD octant . 31 3.7 Three different components of the calorimeter and their polar angle coverages. 33 3.8 Schematic view of a FCAL module in ZEUS. 34 3.9 Luminosity monitoring system at ZEUS . 36 4.1 Examples of leading order diagrams in photoproduction process . 44 vii viii LIST OF FIGURES OBS 4.2 The xγ reconstruction for 1994 dijet analysis in ZEUS . 45 4.3 An illustration of the Breit frame at HERA . 51 jet(1) 4.4 The distribution of the ET as a function of total transverse energy of the event, ET . 54 4.5 An example of fireball-enriched events in ZEUS . 55 4.6 A fireball-depleted event with a clear dijet production in ZEUS . 55 5.1 An example of Λ decay . 59 5.2 Study on the secondary scattering contamination events . 60 5.3 An investigation of the collinearity cut . 61 5.4 The effect of the collinearity cut . 61 5.5 The momentum difference between the decay daughters proton and pion in Λ pπ . 63 ! 5.6 The momentum difference between the decay daughters pions in K 0 S ! π+π− . 63 5.7 Considerations on the cut criteria of the transverse momentum and pseudorapidity . 64 5.8 The invariant mass distribution of candidates assuming a pπ and π+π− mass hypothesis . 65 5.9 The Λ and Λ¯ invariant mass distributions . 66 ¯ 0 5.10 Λ, Λ and KS decay length distributions . 67 0 5.11 The KS invariant mass distribution . 68 ¯ 0 5.12 The Λ, Λ and KS reconstruction efficiencies, purities and acceptances for photoproduction events . ..